The Stowaway in the Bubble

Why it matters

In the lab, hydrogen and nitrogen are collected by bubbling them up through water. Every bubble that rises picks up an invisible stowaway — water vapour — and once inside the jar, that stowaway pushes on the walls just like the gas itself.

Predict first

Moist hydrogen collected over water shows a total pressure of 759 mm Hg. Is the hydrogen itself pushing with 759 mm Hg?

A gas collected over water is moist: P_total = P_gas + P_water vapour. The water vapour's share is called aqueous tension, so the dry gas's actual pressure = total pressure − aqueous tension.

• No — part of that pressure comes from water vapour, so the hydrogen alone exerts less — correct
• Yes — water vapour is far too light to exert any pressure
• No — the hydrogen exerts more, because the water absorbs some pressure

What you do

Sort each statement: does it describe a gas freshly collected over water, or a perfectly dry gas?

Check yourself

A gas is collected over water. Its actual (dry gas) pressure equals…

The moist gas's total pressure is the sum of the dry gas's partial pressure and the water vapour's partial pressure (aqueous tension). So P_gas = P_total − P_water vapour.

Moist nitrogen shows a total pressure of 659 mm Hg at 9°C, where the vapour pressure of water is 9 mm Hg. What pressure does the dry nitrogen alone exert?

Pressure due to dry nitrogen = total pressure − aqueous tension = 659 − 9 = 650 mm Hg. This corrected pressure is the P you must use in gas-law calculations.

Why can gases like hydrogen and nitrogen be collected over water in the first place?

Collection by downward displacement of water only works for gases that water cannot dissolve or react with — hydrogen and nitrogen pass through unharmed. The price you pay is a moist gas whose pressure needs the aqueous-tension correction.

• total pressure − aqueous tension — correct
• total pressure + aqueous tension
• aqueous tension − total pressure
• 650 mm Hg — correct
• 668 mm Hg
• 659 mm Hg
• They are insoluble, or only slightly soluble, in water — correct
• They are lighter than water, and lightness is all that matters
• They react with water to make extra bubbles